Polycrystalline NiAl Research Articles

Combustion-synthesised TiAl/NiAl layered intermetallic composite: synthesis, phase composition, and microstructure

ABSTRACT The joining of different intermetallics (e.g. NiAl, TiAl) is important for high-temperature applications. In this study, self-propagating high-temperature synthesis was used for simultaneous joining of (Ni+Al) and (Ti+Al) layered samples of the stoichiometric compositions. The main advantage of such joining by self-propagating high-temperature synthesis is that it could be done at ambient temperature without the use of a furnace. The applied load during combustion synthesis was used to improve the contact between the Ni-Al and Ti-Al layers. The obtained results demonstrate that TiAl/NiAl intermetallics were successfully joined together with a bonding interface of about 170 μm thickness. XRD analysis has shown that the joining area consists of NiAl, TiAl, Ti3Al, and NiTiAl2 phases. The SEM data analysis of the transition zone of NiAl-TiAl conjunction has revealed that the transition zone consists of several intermediate layers: a polycrystalline NiAl layer; an intermediate layer based on Ni-Al-Ti containing NiTiAl2 phase; a layer based on Ni-Al-Ti with mixed dendritic and eutectic regions; and a polycrystalline TiAl layer.The measured microhardness of the transition zone varies from 3200 up to 13,600 MPa with the maximum microhardness of 1355 ± 45 MPa corresponding to the layer identified as the NiTiAl2 phase.

The importance of grain boundaries on the reactive elements effect in β-NiAlHf alloy

Cyclic oxidation of single crystalline and polycrystalline β-NiAlHf alloy was investigated at 1200 °C. The effect of grain boundary on the role of reactive elements was discussed. The results indicate that Hf effectively reduced the oxide scale growth rate and enhanced the scale adhesion in polycrystalline NiAl. But the single crystalline NiAlHf exhibits a higher oxide scale growth rate, severer internal oxidation and catastrophic scale spallation. The absence of the grain boundary leads to the negative effects of Hf.

The influence of temperature on the mechanical properties of polycrystalline and single-crystalline Ni3Al

Because of the excellent mechanical property at high temperature, single-crystalline Ni3Al-based alloys, which belong to Ni-based superalloys, are widely adopted in the fields of aerospace, aviation and military. Usually, single-crystalline Ni-based alloys are bonded or welded together by polycrystalline Ni-based alloys. In this study, the Young’s modulus, shear modulus, bulk modulus, and Poisson ratio of polycrystalline Ni3Al at the temperature ranging from 10 °C to 850 °C were calculated according to Voigt–Reuss approximation, and the relationships between the mechanical properties and the temperatures were clarified. Moreover, the influence of temperature on the universal anisotropy index and mechanical properties of single-crystalline Ni3Al was evaluated. Results show that, except for Poisson ratio, the increase of temperature linearly decreases the mechanical properties of polycrystalline Ni3Al, while it increases the universal anisotropy index of the single-crystalline Ni3Al. Moreover, different with the bulk modulus, the Young’s modulus and shear modulus of single-crystalline Ni3Al are obvious anisotropic, and their anisotropic characteristics are quite opposite. The increasing temperature decreases both maximum and minimum values of Young’s modulus, shear modulus, and bulk modulus of single-crystalline Ni3Al.

Stress Concentration Induced by the Crystal Orientation in the Transient-Liquid-Phase Bonded Joint of Single-Crystalline Ni3Al

Single-crystalline Ni3Al-based superalloys have been widely used in aviation, aerospace, and military fields because of their excellent mechanical properties, especially at extremely high temperatures. Usually, single-crystalline Ni3Al-based superalloys are welded together by a Ni3Al-based polycrystalline alloy via transient liquid phase (TLP) bonding. In this study, the elastic constants of single-crystalline Ni3Al were calculated via density functional theory (DFT) and the elastic modulus, shear modulus, and Poisson’s ratio of the polycrystalline Ni3Al were evaluated by the Voigt–Reuss approximation method. The results are in good agreement with previously reported experimental values. Based on the calculated mechanical properties of single-crystalline and polycrystalline Ni3Al, three-dimensional finite element analysis (FEA) was used to characterize the mechanical behavior of the TLP bonded joint of single-crystalline Ni3Al. The simulation results reveal obvious stress concentration in the joint because of the different states of crystal orientation between single crystals and polycrystals, which may induce failure in the polycrystalline Ni3Al and weaken the mechanical strength of the TLP bonded joint. Furthermore, results also show that the decrease in the elastic modulus of the intermediate layer (i.e., polycrystalline Ni3Al) can relieve the stress concentration and improve the mechanical strength in the TLP bonded joint.

Molecular dynamics study of high temperature wetting kinetics for Al/NiAl and Al/Ni3Al systems: Effects of grain boundaries

The high-temperature wetting of metal droplets on alloy metal substrates is of practical interest owing to its wide range of applications in brazing, welding and soldering. The surface and bulk of the alloy metals contain very many grain boundaries, whose effects on high-temperature wetting and the associated mechanisms have not been satisfactorily studied. In this work, two spreading scenarios are considered using molecular dynamics simulations to investigate the effects of grain boundaries on the wetting kinetics of the spreading of Al droplets on substrates with (polycrystalline NiAl) or without (monocrystalline NiAl) dissolutive reactions and at various rates of solution (monocrystalline and polycrystalline Ni3Al). Dissolutive reactions occur only when an Al droplet spreads over a polycrystalline NiAl substrate, and they significantly accelerate the wetting kinetics. Irregular atomic alignment and associated with grain boundaries, atomic migration or dissolution reactions occur more easily on grain boundaries than on ideal crystal substrates. Dissolutive reactions occur when Al droplets spread on either monocrystalline or polycrystalline Ni3Al substrates; however, grain boundaries influence the dissolution rates in various spreading stages. The dissolution rate of the Al(l)/polycrystalline Ni3Al(s) system with grain boundaries exceeds that of the Al(l)/monocrystalline Ni3Al(s) system in the initial stage of spreading, but is less than that of the Al(l)/monocrystalline Ni3Al(s) system in the final stage of spreading. Correspondingly, the spreading of the Al(l)/polycrystalline NiAl(s) system is faster than that of Al(l)/monocrystalline NiAl(s) system in the first stage but slower in the last stage. This result confirms that dissolutive wetting is dominated by dissolutive rate rather than by the total dissolutive quantity, and a high dissolution rate accelerates wetting.

Novel positive exchange bias effect in compacted Ni3Al/NiO core/shell nanoparticles

We have observed an unusual type of positive exchange bias (EB) in polycrystalline Ni3Al–NiO core-shell nanoparticle system in that the M–H loop center shifts in the same direction as that of the cooling magnetic field, Hcool, regardless of the sign and magnitude of Hcool. However, for weak (≲ 50 Oe) negative cooling fields, as the temperature is raised, a crossover from positive to negative EB occurs at a temperature, which depends on the cooling temperature and Hcool. This kind of positive EB is not accompanied by the training effect but by the vertical shift in the hysteresis loops, ME. A simple model of single-domain ferromagnetic particles embedded in a disordered antiferromagnetic matrix is shown to provide a straightforward explanation for the direct relation observed between the EB field, HE, and −ME.

Size-dependent plastic deformation and failure mechanisms of nanotwinned Ni3Al: Insights from an atomistic cracking model

The polycrystalline Ni3Al is brittle since the notorious intergranular fracture mode hinders its applications. Here we perform molecular dynamics to highlight the unique role of nanotwin boundary in the plastic deformation and failure mechanisms of Ni3Al via an atomistic cracking model. Surprisingly, the strength, ductility and fracture toughness of the nanotwinned Ni3Al are revealed to increase simultaneously with reducing twin size, possibly evading a traditional tradeoff between ductility/toughness and strength. A possible quasi-brittle fracture mode in single crystalline Ni3Al is recognized as nucleating twinning partials from crack tip. However, the pre-existing twin boundaries can suppress the emission and propagation of successive twinning dislocations. Instead, dislocation avalanches happen and serve as a crack blunting mechanism which leads to the ductile fracture pattern of the nanotwinned Ni3Al. A size-dependent transition of fracture mode from dislocation nucleation to shear localization is observed as twin becomes very small. A physical model combined with energetics analysis is provided to rationalize the transition. Our atomistic insights are in qualitative agreement with recent observations of improved strength and ductility of Ni3Al with disordered nanotwinned structure after severe plastic deformation.

Influence of hydrostatic pressure on texture evolution in HPT deformed NiAl

NiAl is an intermetallic compound with a brittle-to-ductile transition temperature of about 300°C at ambient pressure. At standard conditions, it is very difficult to deform, but fracture stress and fracture strain are increased under hydrostatic pressure (HP). On account of this, deformation at low temperatures is only possible at high HP, as for instance used in high pressure torsion (HPT). In order to study the influence of HP on texture evolution, small discs of polycrystalline NiAl were deformed by HPT at different temperatures ranging from room temperature to 500°C and different HPs. The influence of HP is presented for deformation at room temperature and 500°C. It is found that HP affects the formability of the samples as well as texture and microstructure.

Influence of deformation temperature on texture evolution in HPT deformed NiAl

NiAl is an intermetallic compound with a brittle-to-ductile transition temperature at about 300°C and ambient pressure. At standard conditions, it is very difficult to deform, but fracture stress and fracture strain are increased under high hydrostatic pressure. On account of this, deformation at low temperatures is only possible at high hydrostatic pressure, as for instance used in high pressure torsion. In order to study the influence of temperature on texture evolution, small discs of polycrystalline NiAl were deformed by high pressure torsion at temperatures ranging from room temperature to 500°C. At room temperature, a typical shear texture of body centred cubic metals is found, while at 500°C a strong oblique cube component dominates. These textures can be well simulated with the viscoplastic self-consistent polycrystal deformation model using the primary and secondary slip systems activated at low and high temperatures. The oblique cube component is a dynamic recrystallization component.

Effect of alloying elements on dislocation in NiAl: A first-principles study

The calculation of the ductility criterion, the antiphase boundary energy and the Peierls stress indicate that compared with Cr, Au, Fe and Mn are better alloying elements improving the room-temperature ductility of polycrystalline NiAl. If the site preference behavior of Re, Os, Ir, Pt and Co can be reversed, these elements will also become better ductility elements and the Ni antisite at Al site is beneficial to the ductility of NiAl polycrystalline.

High resolution residual stress measurement on amorphous and crystalline plasma-sprayed single-splats

Residual stress was measured on plasma sprayed crystalline NiAl, Al2O3 and amorphous Al2O3TiO2ZrO2CeO2 single splats, by using an incremental focused ion beam (FIB) micron-scale ring-core method (IμRCM).Tensile residual stress exists in polycrystalline NiAl splats, where the quenching stress is only partially relaxed by edge curling and through-thickness yielding. Significant compressive stress was observed for the amorphous Al2O3TiO2ZrO2CeO2 splats, where viscous flow above the glass transition temperature completely relaxed the quenching stresses without micro-cracking. Comparatively lower compressive stress was measured on crystalline Al2O3 splats, where, in spite of extensive micro-cracking, not all of the tensile quenching stress was relaxed. Using stress data and micro-crack geometry, the intrinsic shear adhesion strength of Al2O3 splats was calculated, giving insights into the role of (sub)micron-scale phenomena on adhesion/cohesion of thermally sprayed coatings. The proposed stress build-up mechanisms and relaxation phenomena are supported by a TEM microstructural analysis of the splats.The experimental methodology developed provided a unique way for the study of the residual stress build-up mechanisms in amorphous and crystalline single splats obtained by plasma spraying, and gave further insights into the actual micro-scale phenomena that give rise to adhesion and nano-mechanical behavior of thermally sprayed coatings.The proposed approach is also expected to find a wide range of applications in materials science and engineering, as it allows for the residual stress measurement even on amorphous materials with micrometer spatial resolution.

Alloying element additions to Ni 3Al: Site preferences and effects on elastic properties from first-principles calculations

First-principles calculations were performed to study the effects of alloying elements (Mo, Re, Ta, W, Ti, Co, Nb, Ru, Cr, Y) on the elastic properties of Ni 3Al. The site preferences of the alloying elements in Ni3Al at different temperature and concentrations were predicted. The influence of alloying elements on the lattice parameters of Ni 3Al were calculated and compared with the values fitted from experimental data. The effects of alloying elements on the elastic constants of Ni 3Al were present. The directional shear and Young’s moduli for single-crystal Ni 3Al alloys with alloying elements were estimated. The bulk, shear, and Young’s moduli of polycrystalline alloys were obtained. It is found that all the alloying elements occupy Al sites except Ru and Co, which may occupy both sites depending on concentration and temperature. All the elements increase shear and Young’s moduli of single-crystal Ni 3Al in all orientations except Cr, Co and Y. All the elements increase both bulk and shear moduli of polycrystalline Ni 3Al except Co and Y. The solute atoms with higher bulk modulus tend higher bulk modulus of Ni 3Al alloys, and the bulk modulus is related to the mole volume either.

Ni3Al and NiAl by XPS

Core- and valence-band levels XPS spectra of Ni3Al and NiAl including the energy loss parts were obtained for an in-situ fractured surface. Polycrystalline Ni3Al and NiAl were prepared by arc melting under argon atmosphere; this was followed by annealing at pressures less than 2.0 × 10−3 Pa.

Equal-Channel Angular Pressing of NiAl

Equal-channel angular pressing (ECAP) was applied to polycrystalline NiAl at temperatures around the brittle-to-ductile transition temperature (BDTT). NiAl rods encapsulated in a steel jacket were ECAP-processed in a die with a channel angle of 120°. The microstructure and texture were characterized by electron backscatter diffraction with a scanning electron microscope. The volume fraction of the texture components typical for simple shear in the intersection plane of the channels changes in the range of the BDTT.

Deformation mechanisms of hard oriented NiAl single crystals

Hard oriented NiAl single crystals (<100> deformation axis) have been compressed at temperatures between 296K and 963K. At about 600K the yield stress changes from a plateau to a steep fall. Slip line investigations show that this coincides with a transition from {112}<111> to {110}<110> slip. At low temperatures slip of <111> dislocations is determined by the Peierls mechanism while in the plateau region the deformation mechanism is still not understood. For the deformation by {110}<110> slip the activation enthalpy measured supports the deformation mechanism proposed by Mills et al. [1] based on the diffusion-assisted motion of macro-kinks in <110> edge dislocations which are found to be decomposed into <100> edge dislocations. The slip transition temperature is discussed with regard to the brittle-to-ductile transition temperature of polycrystalline NiAl.

105 分子動力学法による多結晶Ni-Alの形状記憶シミュレーション(OS1-1 形状記憶材料の特性評価,OS1 固体力学の最前線:実験とシミュレーション)

105 分子動力学法による多結晶Ni-Alの形状記憶シミュレーション(OS1-1 形状記憶材料の特性評価,OS1 固体力学の最前線:実験とシミュレーション)

Influence of the processing route on microstructure and mechanical properties of the polycrystalline NiAl

The influence of grain refinement on the mechanical properties of differently processed NiAl was presented. Five NiAl near-stoichiometric alloys were investigated, three conventionally cast and two sintered from powders (fine-grained powder made by mechanical alloying and nanopowder made by gas condensation). The mechanically alloyed and cast materials were hot extruded at different conditions to obtain diverse grain sizes. The nanomaterial was synthesized in inert gas condensation and then compacted at 1.4 GPa at 300 degrees C. It was shown that the ductility and strength can be directly controlled by appropriate texture and grain refinement.

Positron annihilation in B-doped and undoped single and polycrystalline Ni 3Al alloys

Coincidence Doppler broadening spectra measurements on B-doped and undoped single and polycrystalline Ni 3Al alloys exhibit characteristic differences. The probability of the positron–3d electron annihilations decreases with the increase of B concentration in single crystals of Ni 3Al alloys, while it increases with B concentration in polycrystals of Ni 3Al alloys, as long as the concentration of B is less than its solubility limit. The behavior of positrons and B atoms in single and polycrystals of Ni 3Al alloys has been discussed.

Sputtering-induced nanometre hole formation in Ni3Al under intense electron beam irradiation

The irradiation damage of polycrystalline Ni3Al thin foils of stoichiometric composition by a stationary nanoscale 200 keV field emission gun (FEG) electron probe in a FEI Tecnai F20 (S)TEM has been investigated. At current densities greater than 107 A/m2, nanometre holes are produced quickly with both ⟨001⟩ and ⟨110⟩ incident electron beam directions. EDX spectra from the irradiated volume have been collected simultaneously during the hole forming process. From the EDX results, preferential surface sputtering of aluminium from Ni3Al has been demonstrated. To understand the underlying physical process of sputtering, modelling based on a combination of molecular dynamics and Monte Carlo simulation has been performed. It appears to reproduce faithfully the overall film sputtering and hole formation processes, but is not capable of predicting the detailed geometry of the hole. It predicts that the sputtering cross-section of Al atoms is much higher than that of Ni atoms, resulting in a very small concentration of Al at the surface. This, together with the increase of surface area during hole formation, explains the preferential Al loss observed from the specimen. Calculated sputtering rates agree well with experiment, and are of the order of magnitude of 10−8 atoms/electron.

Thermo Mechanical Treatment of Nickel and Titanium Aluminides

Polycrystalline Ni3Al and TiAl are attractive materials for high temperature structural applications due to their stability in oxidizing and sulphidizing environment upto700 0 C. They possess significantly higher specific stiffness and similar specific strength as that of super alloys. Hence, these materials can replace super alloys for high temperature applications (~900°C). TiAl has lesser density and can be used for reducing component weight up to 50% and suitable for aerospace and automobile (high performance vehicles) sectors. The major difficulty for putting Ni3Al for engineering applications is its extremely low ductility and inter-granular fracture at ambient temperatures. TiAl, apart from the said brittleness it also suffers from high temperature corrosion. However the brittleness of these aluminides can be reduced by micro-alloying and by subjecting them to Thermo Mechanical Treatments, TMT. This paper deals with the recrystallization studies on nickel aluminides, deformed to different extents by rolling. The average grain size dependence with the % elongation is evaluated in the grain size range of 10-35micron. For the nickel aluminide deformed for 50% by rolling, the variation of resistivity and hardness with annealing time is determined. The homogenized TiAl samples were cold worked and annealed at 1000 0 C. Since the aluminide suffers from low ductility at room temperature, an arbitrary parameter, electrical resistivity, was chosen. Corresponding hardness values were also obtained. Finally a qualitative determination of ductility was made by studying the flow behavior of alloy around the hardness indentation. Thus a correlation was developed between resistivity, hardness and ductility values. It was then to some extent possible to investigate the TMT cycles on the microstructure and hence on the ductility of the TiAl without going for the actual tensile tests.